1. Field of the Invention
The present invention relates generally to an innovative electric screwdriver, and more particularly to a two-stage locking electric screwdriver with external torque modulation, featuring convenient modulation, accurate torque, lower power consumption and minimal number of parts.
2. Description of Related Art
The technique of screw (bolt) locking is playing a significant role in various industrial fields and modern technologies. The screw (bolt) assembly technique is particularly important in the automobile, aerospace and bolt manufacturing fields
SCHATZ is a company with more than 50 years of technical experience and has long been engaged in the development of solutions for assembly and quality assurance of various bolts. The company has set a benchmark for the industry, and its technological level has become national or even international standards and guidelines; Doctor Volker Schatz authored a book named 10 Steps for Reliable Bolted Assembly, which described in detail 10 steps affecting all factors in the process of assembly, and analyzed the causal relationship affecting the bolt connecting and fastening quality. These 10 steps can be made into a step-by-step examination chart to monitor perfectness of the methods utilized during the process of bolt connecting and fastening Of course, these 10 steps can also be used in formulating guidelines reflecting state-of-the-art quality control systems and standards. Based on the guide of this book, the locking quality assurance can be enhanced to the top level meeting all current standards and methods.
In this book, there is a description that: in terms of threaded bolt fastening, the tightening method is the most important aspect in the assembly strategy. Each bolt fastening operation to assemble the components shall have a guideline to indicate the fastening steps and the measurement data to be used. For this purpose, the parameters for each assembly step shall be calculated based on an abstract model. Due to the abstract model, these parameters are only an approximation of real conditions. The assumptions and estimations provided here can be used as the basis for the design of fastener connection. On the other hand, during actual assembly operations, those charts, standards and testing results often cannot reflect the factors influencing the assembly, while the results are often used as the basis for the assembly operations. In current fastening standards adopted in the production process, there is usually a specified torque with an appropriate tolerance range, or an indication to use the Turn-of-nut Installation Method to tighten the fastener, i.e., rotating the fastener to a certain angle to reach the preset torque. The bolt connecting and fastening process is seemingly very simple. When using a tool with rotating power for fastening, the bolt is screwed into the screw hole. Not until the screw head touches the assembled piece will the twisting tool suddenly stop rotation. During this process, most of the time is used to screw in the fastener. In actual assembly, i.e., binding two or more parts together, this action is instantly finished through frictional force within 1 second.
Just because the locking assembly process is finished in a very short time, in the locking operation to bind elastic materials, defects are almost unavoidable. As shown in FIG. 1, when the fastener is instantly locked, an elastic deformation occurs. When the elastic material has a plastic deformation, like a spring, even though the screw (bolt) is locked within its elastic limit, it may also cause looseness of the locking, i.e., loss of the so-called clamping force.
Further referring to FIG. 2, the multi-functional bolt fastening analysis system in the German Schatz Laboratory analyzed the performance of the screw (bolt) based on different locking objects. FIG. 2 (A) depicts the performance of the bolt when locking a material thinner than the length of the bolt. FIG. 2 (B) depicts the perforce of the bolt when installed on a material thicker than the length of the bolt.
Based on the descriptions, the above FIG. 2 (B) means that, when the screw (bolt) is rotated in a very instant torque to reach the locking status, even if the torque is maintained within the tolerance range, the locking operation is actually incomplete and the screw (bolt) is not properly installed. On the other hand, FIG. 2 (A) means that, in the case of a two-stage locking, the situation occurred in FIG. 2 (B) can be avoided, and the screw (bolt) locking is properly installed and completely tightened.
Conventional electric screwdrivers can be divided into internal torque modulation and external torque modulation. Referring to FIG. 3 for internal torque modulation, the torsion spring is configured inside (i.e., between the coupling shaft and the torque tube), and is enclosed by a torque sheath. In the operation to modulate the torque, as the torsion spring is located inside the structure, a specially designed torque wrench (or tool) must be used to turn a specific angle to adjust the torque. Once the special wrench (or tool) gets lost, the torque can no longer be adjusted; Although the internal torque modulation is not easily realized, it has such an advantage that: when the motor is started, the no-load current will not be affected by the torque of the clutch. Hence, it is energy-efficient.
Secondary, in the case of external torque modulation, the torsion spring is also configured inside, but there is an external torque modulating ring (mostly commonly seen in the market). In operation, it can be directly modulated by hand and is therefore very convenient; but the shortcoming is: when the motor is started, the no-load current will vary depending on the modulated torque. Hence it consumes more power.
Furthermore, apart from the motor, an ordinary electric screwdriver at least includes a gear unit, a clutch unit and a screwdriver head unit. In either internal torque modulation or external torque modulation, when the screwdriver is started, the motor drives the gear unit, and the gear unit drives the clutch unit and screwdriver head unit to operate. This will increase the rotational weight and will consume more power.